.box {
  display: flex;

  /* style */
  width: 100%;
  height: 800px;
  background-color: lightsteelblue;

}

.item {

  /* style */
  /* flex-grow: 1; */
  /* height: 80px; */
  border: 1px solid darkblue;
  background-color: rgb(177, 245, 198);
}

.item1 {
  flex-grow: 1;
}

.item2 {
  flex-grow: 5;
}

.item3 {
  flex-grow: 1;
}